Beat-to-Beat blood pressure (BP) measurements are commonly achieved in clinical practice by introducing an intra-arterial catheter. This method provides accurate and instantaneous blood pressure (BP) measurements. A disadvantage of this method is that it is invasive and can be used only in clinical practice.
For this reason, methods have been developed in order to achieve an accurate fully continuous and comfortable measurement of arterial blood pressure (BP) in the field of clinical and ambulatory monitoring. However, none of the current state-of-the-art measurements techniques achieve such requirements.
There are currently only two methods that are recognized as standard methods in non-invasive assessment of the blood pressure (BP): the oscillometric technique already introduced in 1876 by Marey and the auscultatory method introduced in 1905 by Korotkoff. It is even more remarkable the fact that Koroktoff's method has little evolved since its invention and it still requires the presence of a well trained observer equipped with a stethoscope. Therefore, with such method, important deviations are observed being related to observer's judgment.
Recently, the oscillometric technique has lead to a generation of low-cost automated blood pressure (BP) monitors boosting the field of ambulatory blood pressure (BP) monitoring. Although such devices allow the self-measurement of blood pressure (BP) from the upper-arm or the wrist even in noisy conditions and during motion, accuracy might be compromised due to wrong body or arm position. Moreover, two main factors limit the real ambulatory use of oscillometric devices: the too long measurement time required (approx. 30 seconds) and the decreased measurement frequency due to comfort issues (15 to 30 minutes).
However, automated oscillometric devices remain the only choice in ambulatory blood pressure (BP) measurement and they are currently used in most of the ambulatory and epidemiological studies. But, because of the very low sampling frequencies of the oscillometeric method (typically, one measurement every 30 minutes), problems related to oversmoothing, aliasing and overmodeling might appear.
Other new methods for the non-invasive assessment of arterial blood pressure (BP) have been developed.
The arterial tonometry is a method based on compressing and partially flattening a superficial artery against its underlying bone. A pressure sensor translates then the intra-arterial forces to arterial pressure waveforms. However, the translation requires a calibration procedure to provide absolute blood pressure (BP) measurements. Some commercial devices are already available in clinical practice and require intermittent calibrations via arm-cuff oscillometric technique. However, aside from the problems related to the sensibility of the approach to motion artefacts, the accuracy of such devices has been continuously contested.
An evolved solution to applanation tonometry has also been introduced by MEDWAVE. By gently compressing and decompressing the radial artery during a period of 12 to 15 beats, a transducer identifies the zero-load state on the artery wall and estimates systolic blood pressure (BP) out of it. However, the device is not able to cope with motion artifacts during the measurement and requires subjects to remain still.
In the Volume-Clamp method, a small cuff incorporating a photo-plethysmograph device is placed on a finger. The diameter of the arteries under the cuff is kept constant (clamped), despite of the changes in arterial pressure during each heart beat, i.e. the pressure exerted by the cuff is pneumatically controlled to track the photoplethysmographic signal in real-time and assure a constant light transmission through the phalanx. The servo-controlled cuff pressure follows then the instantaneous arterial pressure waveform. The accuracy and reproducibility of the method has been demonstrated in several clinical studies. However, evidences exist on problems associated to the automatic recalibration and the overestimation of systolic pressure.
Moreover, the volume-clamp method remains an occlusive method that can lead to periods of venous congestion during prolonged use. Additionally some models have been proposed to reconstruct brachial and central pressure from finger measurements.
Another method is based on the Pulse Wave Velocity (PWV). It is known that the velocity of pressure pulses travelling through an artery is correlated with its elasticity (or stiffness). Already in 1878 Moens and Korteweg independently came out with a close expression relating the velocity of pulse waves (c) in a thin, infinite elastic tube with its mechanical properties. According to their model, c is determined by the relationship
      c    2    =      tE          ρ      ⁢                          ⁢      d                      where c is the so-called Pulse Wave Velocity, t, E, and d are respectively the thickness, the Young's modulus and the diameter of the tube, and p is the density of the fluid in the tube.        
Later on, in 1961, Bergel experimentally determined and quantified that the Young's modulus of an artery in animals is strongly dependent on the internal pressure in the vessel. Hence, one can rewrite the above equation as:
      c    2    =            t              ρ        ⁢                                  ⁢        d              ⁢          f      ⁡              (                  B          ⁢                                          ⁢          P                )                            where BP is the blood pressure, and f (BP) depicts the dependency of E on BP, and it can be experimentally determined. Several authors have considered f (BP) to be an exponential function of BP. This equation shows that by fixing the value of        
      t          ρ      ⁢                          ⁢      d        ,there exist a one-to-one correspondence between c and BP: that is, once the values of
  t      ρ    ⁢                  ⁢    d  and the function f (BP) have been fixed it is possible to obtain blood pressure (BP) from pulse wave velocity (c) measurements. Such method is described for example in U.S. Pat. No. 6,599,251. According to this method:                                    an initial guess on the values of                        
  t      ρ    ⁢                  ⁢    d  and f (BP) is done during a calibration procedure,                                    a sensing methodology continuously computes the velocity of the pulse wave (c) generated at each heart beat through a segment of the arterial tree,            assuming the calibration values to be constant, the measured values of c are used to continuously estimate BP.                        
However, the method describes in U.S. Pat. No. 6,599,251 does not take in account the dynamic adaptation of
      t          ρ      ⁢                          ⁢      d        .Indeed, the three parameters do get continuously modified in a living cardio vascular system. Some recent studies have measured the modifications of diameter in human arteries during different cardio vascular adaptations in-vivo. The change in diameter of radial artery is assessed using an echo-Doppler device: during the recordings, the diameter of the radial artery was found to increase from 2 mm to 2.2 mm (10% increase). With another method measuring the changes in diameter of the carotid artery by an ultrasound imaging technique, it has been found that the diameter is increased from 6 to 7mm (16% increase) when the blood pressure (BP) is increased by 30 mm Hg.
EP 1 344 489 discloses a continuous non-invasive process for measuring blood pressure using impedance plethysmography. This process comprises a step for calculating the blood volume change from the first pulse and a step for calculating the blood volume change from the second pulse. However, if blood pressure derivates due to changes in vessels compliance, a recalibration is needed to compensate for these changes employing a simultaneous measurement of stroke volume and blood pressure measurement. For each recalibration, an oscillometeric blood pressure measurement or a manual auscultatory method is used.
U.S. Pat. No. 5,309,916 discloses a device for measuring the blood pressure using local measurements of pulse wave velocity and blood flow velocity. Accordingly, the local blood pressure of the vessels underneath the sensor means is calculated as a pre-calibrated function of the joint variation of pulse wave velocity and blood flow velocity at the measurement site. Therefore, such a device requires the continuous measurement of blood flow velocity, or equivalently, the continuous measurement of rate of blood flow and the diameters of the vessels involved in the measurements.
DE 102 49 863 discloses a non-invasive blood pressure measurement method in which the difference between a signal measured using an impedance cardiograph and that determined using an optical or acoustic peripheral pulse wave is determined. Such method does not take into account the elasticity of the vessels. Therefore, such a method requires frequent recalibration.